Saturday, 19 January 2013

Schmallenberg...where are we at? Part 2

I have a double interest in Schmallenberg: firstly I'm a virologist involved in SBV research, and secondly my parents' dairy farm is SBV positive. Needless to say, I've been keeping an eye on what's happening.

Our first paper has just come out and seems to have been well received. It's by no means the first paper to come out on SBV, but it does establish a reverse genetics system which can be used to manipulate the virus in various ways. We're also on the way to figuring out various aspects of how the virus causes pathology in animals. Arguably just as important though are all the other papers describing the epidemiology of where the virus is and what proportion of animals have been infected etc.

How to make Schmallenberg virus using reverse genetics. From Varela et al.

As far as we can tell, SBV seems to present problems in two main forms: 1) an acute disease resulting in a drop in milk yield and a rather non-specific illness, 2) problems in utero, most strikingly deformed lambs and calves. It's only really the latter which is ever discussed, presumably as the death and deformity are such graphic and obvious losses. It can't be ignored though. Defra's stance remains that SBV is a low impact disease; try telling that to some of the sheep farmers who will end up with fewer lambs to sell this year.

A SBV induced deformed calf (Farmer's Weekly).

In terms of vaccines, there's one under assessment right now, but it's likely to be a case of closing the door after the horse has bolted. And anyway, it almost certainly won't be ready in time for next spring. The purpose of the vaccine would have been to generate immunity in ewes and cattle before pregnancy. Clearly this is now too late; the animals were naive and exposed to the circulating virus at exactly the right time for the fetus to be infected, resulting in the development of deformed young or fetal resorption, and there have been numerous reports of 'empty' cows and sheep. The true extent of the SBV impact is only now beginning to surface as the lambing season continues, and it's not good. One positive piece of news I've heard is that sheep who were infected prior to becoming pregnant (and were thus immune at the time of conception) are lambing fine. It remains to be seen for sure, but it is reassuring that long-term fertility does not appear to be too badly affected. In effect, the nation's sheep and cows have now become immune, by being infected. Assuming this will leave only limited numbers of animals which the virus can still infect (those uninfected thus far, and on the basis that the immunity is long lasting), the impact is likely to be minimal. Without sufficient naive animals the virus may not be sustained within the animal-vector system.

The overall impact will be hard to calculate: how do you tell what proportion of pregnancy failures are truly due to SBV infection? Virus isolation from every individual case will be difficult given the short lived viraemia. Furthermore, how can you really calculate the loss of milk production when there may be many confounding factors?

Where is it heading? My suspicion at this point in time is that next year won't be anywhere near as bad as we're seeing now. Unaffected newborns will initially receive maternal antibody and thus be protected from immediate infection. Even after maternal immuntiy has waned, the young animals may become infected before they reach mating age. In the case of lambs, the majority will be eaten. A vaccine should be available by then, and SBV has caused enough problems this year that it will have a wide uptake by farmers. Next year then, based on what we know now, SBV probably won't be so much of an issue. For now though, it very much is.